The present invention relates to a method of lithographic printing wherein an imaging material is unrolled from a supply roll, wrapped around a cylinder of a printing press, image-wise exposed and processed by supplying single-fluid ink.
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called computer-to-plate (CTP) method has gained a lot of interest. This method, also called direct-to-plate method, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. A special type of CTP processes involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of an image-setter that is integrated in the press. This method may be called xe2x80x98computer-to-pressxe2x80x99 and printing presses with an integrated plate-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science and Technology""s 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364. EP-A 640 478 describes a digital press with an automatic plate-loading system comprising a supply roll and an uptake roll within the plate cylinder.
Typical plate materials used in computer-to-press methods are based on ablation. A problem associated with ablative plates is the generation of debris which is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. Other methods require wet processing with chemicals which may damage or contaminate the electronics and optics of the integrated image-setter and other devices of the press. Therefore, lithographic coatings which require no wet processing or may be processed with plain water, ink or fountain is especially desired in computer-to-press methods. WO 90002044, WO 91008108 and EP-A 580 394 disclose such plates, which are, however, all ablative plates having a multi-layer structure which makes them less suitable for on-press coating. U.S. Pat. No. 6,095,048 discloses the processing of an ablation-type material with a single-fluid ink.
A non-ablative plate which can be processed with fountain and ink is described in EP-B 770 494. The latter patent specification discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby converting the exposed areas into an hydrophobic phase which define the printing areas of the printing master. The press run can be started immediately after exposure without any additional treatment because the layer is processed by interaction with the fountain and ink that are supplied to the cylinder during the press run. So the wet chemical processing of these materials is xe2x80x98hiddenxe2x80x99 to the user and accomplished during the first runs of the printing press.
A problem associated with the latter method is that the on-press processing is done by the steps of first supplying fountain to the plate and subsequently also ink, which can easily be carried out in printing presses wherein the ink and fountain rollers can be engaged independently from one another. However, it is more difficult to optimize on-press processing by the simultaneous application of fountain and ink, which is the only option in printing presses which are equipped with an integrated ink/fountain supply.
In addition, processing of plate materials by fountain is not possible in a driographic press since only ink is supplied to the plate in such presses. Driographic presses need careful temperature control because there is no cooling effect from an aqueous fountain liquid.
So there is need for a method wherein on-press processing of an imaging material can be achieved without the supply of aqueous fountain liquid.
It is an object of the present invention to provide a method of lithographic printing with automatic plate-loading and on-press exposure which enables on-press processing in all lithographic printing presses, also those that contain no fountain supply. In that way, a fully automatic printing method is obtained wherein all the steps of plate-loading, exposure and processing can be carried out without human intervention.
This object is realized by the method of claim 1. It has been found that excellent results can be obtained by using a single-fluid ink for the on-press processing of an imaging material comprising an image-recording layer which is soluble in such a single-fluid ink or can be rendered soluble in the single-fluid ink by the exposure step. Single-fluid ink is generally understood as an emulsion of an ink phase in a polar phase, or vice-versa, an emulsion of a polar phase in an ink phase. Single-fluid ink allows printing with a conventional, wet lithographic printing master without the application of a dampening liquid. The ink phase adsorbs onto the hydrophobic areas of the printing master and the polar phase wets the hydrophilic areas, thereby preventing adsorption of the ink component on the non-printing portions of the lithographic image.
The printing method of the present invention is a cycle of steps (i) through (vi) as defined in claim 1 and this cycle can be repeated several times, the exact number being dependent on the length of the web of the imaging material that is present on the supply roll. Preferably the number of print cycles is larger than 1, more preferably larger than 10 and most preferably larger than 30. Since plate changing and loading is fully automatic, the press down time between print cycles is minimized.
Further objects of the present invention will become apparent from the detailed description. Specific features for preferred embodiments of the invention are set out in the dependent claims.